EP1829147A2 - Solution servant a ameliorer des composants de cellules electrochimiques et d'autres systemes et dispositifs electrochimiques - Google Patents

Solution servant a ameliorer des composants de cellules electrochimiques et d'autres systemes et dispositifs electrochimiques

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Publication number
EP1829147A2
EP1829147A2 EP05848149A EP05848149A EP1829147A2 EP 1829147 A2 EP1829147 A2 EP 1829147A2 EP 05848149 A EP05848149 A EP 05848149A EP 05848149 A EP05848149 A EP 05848149A EP 1829147 A2 EP1829147 A2 EP 1829147A2
Authority
EP
European Patent Office
Prior art keywords
solvent
polymer
electrode
component
membrane
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Application number
EP05848149A
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German (de)
English (en)
Inventor
Jay R. Sayre
Megan E. Sesslar
James L. White
John R. Stickel
Mark C. Stasik
Bhima R. Vijayendran
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Battelle Memorial Institute Inc
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Battelle Memorial Institute Inc
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Application filed by Battelle Memorial Institute Inc filed Critical Battelle Memorial Institute Inc
Publication of EP1829147A2 publication Critical patent/EP1829147A2/fr
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/8663—Selection of inactive substances as ingredients for catalytic active masses, e.g. binders, fillers
    • H01M4/8668—Binders
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/8605—Porous electrodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/8647—Inert electrodes with catalytic activity, e.g. for fuel cells consisting of more than one material, e.g. consisting of composites
    • H01M4/8652—Inert electrodes with catalytic activity, e.g. for fuel cells consisting of more than one material, e.g. consisting of composites as mixture
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/88—Processes of manufacture
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/88—Processes of manufacture
    • H01M4/8825—Methods for deposition of the catalytic active composition
    • H01M4/8828—Coating with slurry or ink
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1007—Fuel cells with solid electrolytes with both reactants being gaseous or vaporised
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • H01M8/1018—Polymeric electrolyte materials
    • H01M8/102—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer
    • H01M8/1023—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer having only carbon, e.g. polyarylenes, polystyrenes or polybutadiene-styrenes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • H01M8/1018—Polymeric electrolyte materials
    • H01M8/1039—Polymeric electrolyte materials halogenated, e.g. sulfonated polyvinylidene fluorides
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • H01M8/1018—Polymeric electrolyte materials
    • H01M8/1069—Polymeric electrolyte materials characterised by the manufacturing processes
    • H01M8/1081—Polymeric electrolyte materials characterised by the manufacturing processes starting from solutions, dispersions or slurries exclusively of polymers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1004—Fuel cells with solid electrolytes characterised by membrane-electrode assemblies [MEA]
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • This invention relates in general to electrochemical systems and devices, and in particular to fuel cell components.
  • a fuel cell is an electrochemical "device” that continuously converts chemical energy into electric energy (and some heat) for as long as fuel and oxidant are supplied.
  • Fuel cells are evolving. Some currently known categories of fuel cells include polymer electrolyte membrane (PEM), alkaline, phosphoric acid, molten carbonate, solid oxide, and biobased. All of these fuel cell types have the advantages of silent operation, high efficiency and zero emission capability. PEMs, however, offer several distinct advantages over the others. Some of these are low temperature operation (80-150 C), quick-start-up, compactness, and orientation independence.
  • PEM polymer electrolyte membrane
  • MEA membrane electrode assembly
  • FIG. 1 shows a detailed schematic of a PEM fuel cell.
  • Fuel cells have been around since 1839, but they have been hindered by component materials which are high in cost and suffer from poor durability. Nevertheless, they have attracted much interest in recent years for their ability to produce electricity and heat with higher efficiency and lower emissions than conventional energy technologies. However, the cost of fuel cells is still too high and technical breakthroughs are required before broad commercial application can become a reality.
  • This invention relates in general to components of electrochemical devices, and to methods of preparing the components.
  • the components and methods include the use of a composition comprising an ionically conductive polymer and at least one solvent, where the polymer and the solvent are selected based on the thermodynamics of the combination.
  • the invention relates to a component for an electrochemical device which is prepared from a composition comprising a true solution of an ionically conductive polymer and at least one solvent, the polymer and the at least one solvent being selected such that
  • the invention relates to a method of improving at least one property of a component for an electrochemical device or at least one property of the electrochemical device, the method comprising preparing the component from a composition comprising a true solution of an ionically conductive polymer and at least one solvent, the polymer and the at least one solvent being selected such that
  • Figure 1 is a cross-sectional schematic diagram of a conventional PEM fuel cell.
  • Figure 2 is a schematic view of an electrospinning setup.
  • Figures 3 and 4 are scanning electron micrograph images of ionomer fibers produced by an electrospinning process.
  • Figure 4 is an enlarged view of a portion of the image in Figure 3.
  • Figure 5 is a typical polarization curve for a PEM fuel cell.
  • Figure 6 is a representative polarization curve of an electrode according to the invention compared to a state of the art electrode.
  • Figure 7 is a close-up of an activation polarization region comparing an electrode according to the invention with a state of the art electrode.
  • Figure 8 is a three-phase interphase schematic for a single catalytic site at the cathode of a PEM fuel cell.
  • Figure 9 is a field-emission SEM image of a Pt/C electrode.
  • Figure 10 is a graph comparing the durability of an electrode according to the invention with a state of the art electrode.
  • Figure 11 shows AFM tapping mode images of electrodes fabricated from «-butyl acetate according to the current state of the art.
  • Figure 12 shows AFM tapping mode images of electrodes fabricated from /-butyl alcohol according to the present invention.
  • Figure 13 is a representative polarization curve comparing one MEA with a high aspect ratio ionomer fiber (according to the invention) and one without.
  • the present invention uses a novel solution thermodynamics approach to selecting an ionically conductive polymer and at least one solvent for preparing a component of an electrochemical device, such as a fuel cell.
  • the thermodynamic approach is described in more detail below.
  • the component prepared according to the invention may include improvement(s) compared to components prepared by conventional methods.
  • the present invention may enhance fuel cell performance and/or durability by engineering the three-phase interphase of the MEAs through 1) the formulation of novel ionomer binder solutions; and, optionally, 2) the development of novel, high aspect ratio ionomer fibers to be used as precursors for electrode and/or MEA fabrication.
  • high aspect ratio means fibers having an aspect ratio within a range of froma bout 100: 1 to about 1000:1 (length: diameter).
  • the invention may create an optimized, multifunctional, nanostructured architecture which reduces polarization losses (reaction rate, resistance and mass transport losses) and/or catalyst loadings.
  • ionomer and “ionically conductive polymer” are used interchangeably herein to refer to a polymer having any significant proportion of ionizable and/or ionic groups.
  • ORR oxygen reduction reaction
  • the dominant polarization losses in a hydrogen-air fuel cell are due to the poor kinetics of the oxygen reduction reaction (ORR) and the resulting transport limitations of the protons and reactants at the cathode.
  • ORR oxygen reduction reaction
  • An understanding of the interactions at the three-phase interphase has allowed the development of high- performance electrodes and MEAs that may reduce the polarization losses, while increasing efficiencies, energy densities and durability.
  • the experimental details regarding MEA fabrication, single-cell testing and characterization i.e.
  • the components prepared according to the invention are important to fuel cells because they may cross all applications, fuels and chemistries. They also may have applications beyond fuel cell technologies, such as ionic polymer metallic composite (IPMC) actuators/sensors.
  • IPMC ionic polymer metallic composite
  • catalysts it should be recognized that the invention is not limited to catalysts per se, but it may also be applicable more generally to metals/inorganics (including salts, oxides and metal alloys) which may or may not facilitate the electrochemical reaction.
  • the invention includes a novel solution thermodynamics approach to select an ionomer and at least one solvent for use in a composition to prepare a component of an electrochemical device, such as an ionomer binder solution used to prepare an electrode.
  • the composition is a true solution, not a dispersion or a colloidal suspension.
  • the true solution is a single phase.
  • a dispersion would consist of at least two phases with an interface between the dispersed and continuous phases.
  • ⁇ G ⁇ H-T ⁇ S; ⁇ 0 for solubility, where ⁇ H is the change in enthalpy and
  • T ⁇ S is the product of temperature, T, and the change in entropy, ⁇ S.
  • ⁇ H is related to the Hildebrand solubility parameter, ⁇ , where
  • the results of the equation could also be ⁇ 0.
  • This parameter represents the total van der Waals force.
  • dispersion forces induced dipole- induced dipole or London dispersion forces
  • polar forces dipole-dipole forces
  • hydrogen bonding forces The Hildebrand value for a solvent mixture can be determined by averaging the Hildebrand values of the individual solvents by volume.
  • Electrodes may include manipulating the ionic functionality, hydrophobicity and/or porosity of the electrode for improved water management (to reduce mass transport losses) while optimizing the triple phase boundary (three-phase interphase) where the hydrogen oxidation reaction and the oxygen reduction reaction can only occur at localized regions where electrolyte, gas and electrically connected catalyst regions contact.
  • One way to accomplish this is through various additive technologies. These could be fluorine-based or inorganic additives (e.g. heteropoly acids, zirconium phosphate, etc.).
  • Two different catalyst ink formulations were prepared. Each contained a standard catalyst : Nafion ® ratio of 2.5:1 (28 weight percent ionomer binder) using 5 weight percent Nafion ® 1100 EW solution (ElectroChem, Inc.) and 20 weight percent Pt on Vulcan XC-72 carbon black (E-tek DeNora). To one formulation, t-butyl alcohol (Aldrich) was added as a diluting solvent, and, to the other, n-butyl acetate (Aldrich) was added. Formulations were allowed to stir on a stir plate overnight at room temperature. The added weight of these chemicals was equal to the weight of the Nafion ® binder solution in each formulation.
  • t-butyl alcohol Aldrich
  • n-butyl acetate Aldrich
  • sonication was used to aid in the dispersion of the electrocatalyst particles. It should be noted that varying ionomer binder loadings (i.e. catalyst : ionomer binder) could be used.
  • catalyst ionomer binder
  • Five (5) cm transfer decals were prepared from glass-reinforced polytetrafluoroethylene (PTFE) films (Saint-Gobain). Each catalyst ink coat / layer was painted on one side of each decal with a flat brush (Winsor Newton) with the appropriate catalyst ink under infrared heat to a final dry weight containing -0.2 mg catalyst per cm" electrode.
  • PTFE polytetrafluoroethylene
  • Nafion ® 112 (Aldrich) was converted to the salt form, then MEAs were fabricated using the following procedure on a Carver hydraulic press: (1) Place MEA assembly into pre-heated (210 C) press and compress at 400 psig for 10 minutes. Note: This temperature and pressure may be higher depending on the type of membrane and binder material. (2) Cool under pressure to room temperature. (3) Remove from press. (4) Peel decals away from the MEA assembly one at a time leaving only the electrodes fused to the membrane.
  • the ionically conductive polymers for use in the invention may be any that are currently known or developed in the future.
  • Some general categories of ionically conductive polymers may include the following. Canonical: Nafion ® ⁇ poly(TFE-co- perfluorosulfonic acid). A sulfonated version of almost any poly(aromatic), such as Radel ® , Kraton ® , PBI, etc. Other acid groups applied to the above: sulfonimides, phosphonic acids, etc. Supported versions of the above: Gore-TEX ® etc. used as supports. Polymers with imbibed solid or liquid acids, such as PBI/phosphoric acid (CWRU ® ) or phosphotungstic acid.
  • Canonical Nafion ® ⁇ poly(TFE-co- perfluorosulfonic acid).
  • PCT/US03/03864 filed Feb. 6, 2003, entitled “Polymer Electrolyte Membranes Fuel Cell System”; and PCT App. No. PCT/US03/03862, filed Feb. 6, 2003, entitled “Polymer Electrolyte Membranes for Use in Fuel Cells”.
  • Other polymers that may be used are disclosed in U.S. Patent No. 6,670,065 B2, issued December 30, 2003, U.S. Patent No. 6,893,764 B2, issued May 17, 2005, and U.S. Patent Application Publication No. 2005/0031930 Al, published February 10, 2005. Further polymers that may be used are disclosed in United States Provisional
  • the solvent(s) for use in the invention may be any that are currently known or developed in the future that are suitable for preparing component(s) of electrochemical devices such as fuel cells.
  • Some examples of typical solvents and diluting agents (co-solvents) used for both Nafion ® and non-Nafion ® (i.e. hydrocarbon) ionomers are shown in Table 1.
  • polymer(s) and solvents(s) used in this embodiment of the invention may be the same as those described above, or they may be different.
  • This setup could be modified by laying the syringe in a horizontal position and using a syringe pump to deliver the ionomer solution to the syringe needle tip. The target would also be relocated so that it would be perpendicular to the syringe needle.
  • This setup could be further modified with vacuum-assist if higher boiling point solvents, such as N-methyl-2-pyrrolidone (NMP), are used,
  • NMP N-methyl-2-pyrrolidone
  • the fibers produced according to the above-described method! are usually continuous fibers. It is also believed that non- continuous fibers may be electrospun and produced in accordance with the present invention to produce a mat, using techniques such as are taught in U.S. Pat. No. 6,252, 129, issued June 2.6, 2001, to Coffee (incorporated by reference herein).
  • the catalyst ink formulation was prepared as discussed previously using ?-butyl alcohol as a co-solvent.
  • Single MEAs could be fabricated primarily by two different techniques utilizing this technology.
  • Another technique would be spinning the fiber directly onto a PEM by utilizing a copper frame with an aluminum backing. The PEM would be placed on fop of the aluminum backing and held in place with a copper frame that comes into contact with the perimeter of the PEM.
  • PTFE masking could also be utilized in this type of fixture where electrical insulation is needed.
  • a catalyst ink or electrode could be applied to the membrane / fiber.
  • the invention allows the production of a composite electrode where the fiber serves as both the reinforcement and as the matrix for the catalyst.
  • the catalyst ink could be electrostatically co-sprayed with the ionomer fiber.
  • the catalyst particles would be attracted to the positively-charged fiber via electrostatic attraction to form an electrode (e.g. the anode).
  • the catalyst ink could alternatively be used without an ionomer binder.
  • the catalyst could be substituted with PtRu/C, Pt-black, PtRu-black, and other precious / non-precious metal catalysts.
  • Single- and multi-walled carbon nanotubes could also be used in these formulations to boost electrical conductivity.
  • the electrocatalyst could be encapsulated within the ionomer fiber during the electrospinning process by including the electrocatalyst in the initial ionomer solution formulation.
  • the catalyst ink could be sprayed (may or may not be electrostatic) in concert with fiber formation (parallel operation), after fiber formation (series operation) or a combination of both.
  • the spraying of the catalyst ink could be halted so that the PEM could be processed entirely by electrospinning / electrospraying.
  • the spraying of the catalyst ink could be re-engaged to produce another electrode (e.g. the cathode).
  • This process could be performed in an iterative fashion to fabricate a continuous stack of MEAs with a single, high-aspect ratio polymer electrolyte fiber (PEF).
  • PEF polymer electrolyte fiber
  • Single-cell testing was performed using a 600 W Fuel Cell Technologies, Inc. test station. This station is equipped with an Agilent Technologies 120 A load module, digital mass flow controllers, an automated back pressure system, 5 cm 2 fuel cell hardware, an on-board AC impedance system and humidity bottle assemblies. The on-board electrochemical impedance spectroscopy system was utilized to measure the in situ high frequency resistance (HFR) of each MEA at a frequency of 2 kHz. The HFR is the sum of the membrane, interfacial and electrode resistances.
  • All MEAs were conditioned at 80 C, 100 percent relative humidity (RH) at 0.50 V for at least 2 hours before polarization curves were collected. Polarization curves were collected from 1.00 to 0.00 V at 0.05 V increments with a 30 second delay.
  • RH percent relative humidity
  • Atomic Force Microscopy Tapping mode atomic force microscopy (AFM) was performed with a Digital Instruments Dimension 3000 scanning probe microscope with a Nanoscope IV controller. A tapping mode tip made of etched single crystal silicon with a nominal tip radius of curvature of 5-10 nm was used during scanning. All samples were kept under desiccant for 24 h prior to analysis. The samples were then scanned immediately at room temperature within a 5 ⁇ m 2 sample area.
  • AFM Atomic Force Microscopy
  • the electrochemical performance of a fuel cell is typically determined by analyzing a polarization curve (cell potential versus current density) as shown in Figure 5.
  • This curve shows a typical fuel cell operation where numerous irreversible losses contribute to overpotentials which cause the cell potential to drop significantly below the theoretical (ideal) value of 1.23 V at 25 C as determined by the Nernst equation. The same holds true for the polarization losses compared to the open circuit voltage during an experimental run. Voltage losses differ between theoretical and experimental equilibrium cell voltages due to the cathode mixed potential between O 2 reduction and H 2 oxidation from crossover to Pt/ C.
  • the initial decrease is associated with the activation polarization region where reaction rate losses at the electrocatalyst dominate due to the sluggish reaction kinetics and low catalyst activity. This is followed by a linear drop in cell potential due to resistance losses (i.e. ohmic polarization). Resistance losses are a combination of the resistance to the flow of electrons through the electrodes and interconnects and the resistance to the flow of ions through the electrolyte.
  • concentration polarization is generally due to mass transport limitations of reactants to the catalytically-active sites.
  • Ionomer Binder Solution Formulation Representative polarization curves comparing an electrode according to the invention (Y-butyl alcohol formulation) to the current state of the art (77-butyl acetate formulation) are shown in Figures 6 and 7.
  • Figure 6 captures the polarization region down to 0.4 V, which makes it relatively easy to discern the resulting overpotential over a range of cell potentials.
  • Figure 7 shows a close-up of the activation polarization region.
  • the Battelle electrode (Y-butyl alcohol formulation) has a current density of 0.6760 A per cm with a high frequency resistance (HFR) equal to 0.06 ⁇ -cm while the state- of-the-art electrode ( «-butyl acetate formulation) has a current density of 0.3996 A per cm 2 with a HFR equal to 0.08 ⁇ -cm 2 .
  • Catalytic sites within fuel cell electrodes maintain a three-phase interphase to be effective. This interphase allows for electronic and ionic continuity while providing access to fuel or oxidant. See Figure 8 for a detailed schematic of a single catalytic site at the cathode.
  • Figure 9 shows a field- emission scanning electron microscope (SEM) image of an actual Pt catalyst supported on carbon black (Pt/C) electrode, which comprises the continuity phase. The larger open areas provide access for fuel or oxidant, which is the second phase. The ionomer binder and membrane, which serve as the third phase, are absent from this photo so that the Pt/C structure would be more apparent. It is desirable to satisfy all three conditions for as many catalytic sites as possible through electrode engineering to offer improvements in MEA performance.
  • Nafion ® forms one of three states when mixed with organic solvents: (i) solution, (ii) colloidal dispersion, and (iii) precipitate.
  • a true solution is formed when the dispersed phase is molecularly dispersed, whereas in a colloidal dispersion the dispersed phase, or colloid, is larger than the molecule.
  • the colloids are characteristic of dimension, however, they are also evenly dispersed throughout the dispersion medium (i.e. solvent).
  • a typical scale for colloids is between 1 and 1,000 nm. As these colloids become larger they will either rise to the surface or fall out of solution forming a precipitate depending on the relative specific gravities.
  • Nafion-based polymers and certain solvents under certain conditions these states can be classified by the dielectric constant, ⁇ .
  • the dielectric constant is used as a rough indication of the solvent's polarity.
  • electrodes prepared according to the invention may have improved performance as measured by a current density of at least about 0.6 A/cm 2 at a cell potential of 0.7 V with a high frequency resistance less than 0.08 ⁇ -cm 2 , under the conditions shown in Figure 6.
  • the electrodes may have improved performance over time as measured by a current density of at least about 0.6 A/cm 2 at a cell potential of 0.7 V with a high frequency resistance less than 0.08 ⁇ -cm 2 , and a current density of at least about 0.04 A/cm 2 at a cell potential of 0.85 V with a high frequency resistance less than 0.08 ⁇ -cm 2 .
  • membrane electrode assemblies prepared according to the invention provide a surprising durability benefit. When an electrode of the assembly is produced using a true solution according to the invention, this has the effect of improving the durability of the polymer electrolyte membrane of the assembly.
  • Figure 10 shows a membrane of an MEA according to the invention having superior durability compared to the state of the art in an open circuit voltage experiment.
  • the invention provides a membrane electrode assembly wherein at least one of the electrodes is prepared from a composition including an ionomer and at least one solvent selected as described above, the membrane of the assembly having improved durability as measured by an open circuit voltage (OCV) holding time of at least about 100 hours under the conditions shown in Figure 10, and preferably at least about 300 hours.
  • OCV open circuit voltage
  • the invention provides a method of improving the durability of a second component of an electrochemical device by preparing a related first component of the electrical device from a true solution according to the invention.
  • the invention also provides a method of improving the durability of a component of an electrochemical device by preparing the component from the true solution according to the invention.
  • the invention provides a method of improving one or more properties of an electrochemical device or its components by preparing at least one component of the electrochemical device with a true solution according to the invention.
  • the invention can apply to any types of components, such as membrane electrode assemblies, membranes, electrodes and/or gas diffusion layers. Beyond high performance and durability, another typically preferred property of an electrode structure is high catalyst utilization.
  • the in situ ECA within the fuel cell was measured by cyclic voltammetry using the area under the hydrogen adsorption/ desorption peaks. These results are shown in Table 4.
  • the ⁇ -butyl alcohol system produced an electrode structure with a significantly reduced ECA range compared to the 77-butyl acetate system.
  • the electrode according to the invention has a low catalyst utilization while still having an improved performance.
  • the electrode has a low catalyst utilization as measured by an electrochemical area of less than about 40 square meters of catalyst per gram of catalyst, sometimes less than about 35 square meters of catalyst per gram of catalyst, and the electrode has improved performance as measured by a current density of at least about 0.6 A/cm 2 at a cell potential of 0.7 V with a high frequency resistance less than 0.08 ⁇ -cm 2 .
  • AFM was utilized to examine the surface morphology of the electrodes. Tapping mode amplitude and phase images of the electrodes were recorded under ambient conditions on a 5 ⁇ m x 5 ⁇ m scale in order to investigate the relative differences in surface morphology of the materials as shown in Figures 11 and 12.
  • Figure 11 it can be seen that there are relatively large polymeric islands or domains present which disrupt the surface morphology and thus the continuity of the three- phase interphase in the r ⁇ -butyl acetate system. These islands range from about 0.63 ⁇ m to as large as 2.50 ⁇ m in length.
  • the ⁇ -butyl alcohol system offers an electrode morphology that appears to be closer to a percolation threshold compared to the ⁇ -butyl acetate system, where there is more of an optimum balance and distribution between the components that provide ionic transport, oxygen diffusion, water transport and an electrochemically active surface area. This also allows greater adhesion to the membrane substrate due to the possible increase in polymer surface area that comes in contact with the membrane. These islands or domains are typically smaller than 0.63 ⁇ m in any one direction.
  • the electrode has a morphology that includes domains of the polymer, where at least about 80% of the polymer domains are smaller than about 0.63 ⁇ m in any one direction, and preferably substantiall all of the polymer domains are smaller than this size.
  • the polymer domains may be present on the surface of the electrode and/or they may be present throughout the electrode.
  • the solubility of the blends can change during electrode drying because of the difference in evaporation rates of the individual components. Azeotropic mixtures, a mixture of two or more liquids that has a constant boiling point, can aid in this process.
  • Some advantages of the present invention may include: increased energy densities and efficiencies; reduced polarization losses; and reduced catalyst loadings.
  • advantages during MEA fabrication may include: enhanced solvent removal (due to vapor pressures of the solvents); paintability; faster processing due to enhanced evaporation rate; more precise control of catalyst loadings (layer-to-layer consistency); enhanced electrode adhesion, and no need for viscosity modifiers (e.g. glycerol).
  • the Battelle invention (with ionomer fiber) has a current density of 0.545 A per cm 2 with a HFR equal to 0.09 ⁇ -cm 2 while the one without an ionomer fiber has a current density of 0.395 A per cm with a HFR equal to 0.06 ⁇ -cm .
  • the present invention may allow the production of PEM-based electrodes that function at >100° C and ⁇ 50% RH.
  • the invention may also allow the production of a direct methanol fuel cell (DMFC) electrode and electrode assembly with decreased MeOH crossover, higher operating MeOH concentration, decreased flooding and increased durability.
  • DMFC direct methanol fuel cell
  • the electrode and the polymer electrolyte membrane are both made with the same type of ionomer fiber. This embodiment provides several advantages, such as no mismatch of coefficients of thermal expansion, and thus minimized residual stresses throughout the MEA.
  • An improved MEA is provided where the electrodes and PEM can shrink/swell in concert with each other.
  • Electrodes are produced that now match higher temperature, lower humidity performing polymer membranes made with materials as described above [0083] Some other advantages of the invention may include a highly-tailorable electrode in regards to solid-state and surface chemistry to optimize the interphase.
  • the electrode may be mechanically tough with the ability to impart flexibility and blunt cracks.
  • the electrode may have improved water management to behave similarly to the Nafion ® -alternative PEM.
  • An MEA prepared according to the invention may have improved mechanical and electrostatic bonding.

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  • Chemical & Material Sciences (AREA)
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  • General Chemical & Material Sciences (AREA)
  • Electrochemistry (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Sustainable Energy (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Sustainable Development (AREA)
  • Composite Materials (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Dispersion Chemistry (AREA)
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  • Fuel Cell (AREA)
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Abstract

L'invention concerne des composants de dispositifs électrochimiques et des procédés pour les préparer. Ces composants et ces procédés comprennent l'utilisation d'une composition contenant un polymère conducteur d'ions et au moins un solvant, le polymère et le solvant étant sélectionnés sur la base de la thermodynamique de la combinaison. Dans un mode de réalisation, un composant de dispositif électrochimique est préparé à partir d'une composition contenant une solution vraie d'un polymère conducteur d'ions et d'au moins un solvant, le polymère et le solvant étant sélectionnés de telle sorte que (δ solvant- δ soluté) < 1, le δ solvant étant le paramètre de solubilité de Hildebrand du solvant et le δ soluté étant le paramètre de solubilité de Hildebrand du polymère. Dans un autre mode de réalisation, l'invention concerne un procédé pour améliorer au moins une propriété d'un composant de dispositif électrochimique ou au moins une propriété de dispositif électrochimique, ce procédé consistant à préparer un composant à partir d'une composition contenant une solution vraie de polymère conducteur d'ions et d'au moins un solvant, le polymère et le solvant étant sélectionnés de telle sorte que (δ solvant- δ soluté) < 1, le δ solvant étant le paramètre de solubilité de Hildebrand du solvant et le δ soluté étant le paramètre de solubilité de Hildebrand du polymère.
EP05848149A 2004-11-16 2005-11-16 Solution servant a ameliorer des composants de cellules electrochimiques et d'autres systemes et dispositifs electrochimiques Withdrawn EP1829147A2 (fr)

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WO2006055652B1 (fr) 2007-05-10
KR20070086110A (ko) 2007-08-27
JP2012216552A (ja) 2012-11-08
US20120276470A1 (en) 2012-11-01
US20080248362A1 (en) 2008-10-09
WO2006055652A2 (fr) 2006-05-26
WO2006055652A3 (fr) 2007-03-29
US8124260B2 (en) 2012-02-28
CA2587729C (fr) 2014-06-10
CA2587729A1 (fr) 2006-05-26
JP2008521174A (ja) 2008-06-19
US8481184B2 (en) 2013-07-09

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